The present invention relates to a ball screw apparatus, and in particular a technique effective for realizing more improvement of low noises, operationability and endurance.
For example, the ball screw apparatus is, as shown in FIGS. 8 and 9, composed of a screw shaft 1 and a nut 2. The screw shaft 1 has a first spiral screw groove 3 on an outer circumferential face thereof. The nut 2 has a second screw groove 4 on its inner circumferential face in opposition to the first screw groove 3 defined in the screw shaft 1. The nut 2 is meshed with the screw shaft 1 through a plurality of balls 5 rotatably charged in a spiral ball rolling path T defined between the first and second screw grooves 3 and 4. The ball rolling path T is provided at its one end with a ball circulating path J for taking up the balls rolling and tracking in the ball rolling path T and sending to the other end. If relatively rotating the screw shaft 1 and the nut 2, and moving one of them in an axial direction, this ball screw apparatus endlessly performs relatively spiral movement between the screw shaft 1 and the nut 2 through rolling of the plural balls 5.
In the ball screw apparatus of such a structure, the ball rolling path T has a face of, for example, Gothic arch, and the ball 5 rolls under an angular contact with the T-face of the ball rolling path at two to four points, while the ball rolling path T spirally continues and by having a return mechanism of the ball circulating path J, change from an on-loaded condition to a loaded condition happens. Since the ball 5 under the loaded condition does not form an ideal rolling condition but creates a spin phenomenon, the ball is easy to cause slip and a ball abrasion occurs. Further, since the adjacent balls 5 contact under a condition of rolling in opposite directions each other, slip is caused between the loaded balls 5. As a result, the balls 5 are prevented from free rolling, and there arise various problems of bad working of the balls, creation of abrasion and damage, fluctuation of torque, or occurrence of noises.
Particularly, in the ball screw apparatus effected on and off with high load applied to an injection molding machine or pressing operation, not only osculating ellipse is large owing to elastic deformation between the ball 5 and the ball rolling path T, but also competition of the ball 5 particular to the ball screw apparatus occurs, so that the abrasion and damage appear more remarkably.
For solving these problems, as shown in FIG. 9, such a measure has been broadly practiced where the retaining pieces 10 having recesses 11 at opposite to the balls 5 are interposed between the adjacent balls 5 rolling and tracking in the ball rolling path T, and this measure enables to suppresses competition of the balls 5.
However, by interposing the retaining pieces 10, a new problem arises that the retaining piece 10 and the ball 5 compete against each other, so that the retaining pieces 10 are effected with abrasion and damage. Abrasion and damage of the retaining piece 10 create spaces around the retaining piece 10, and a sum total in spaces of the retaining pieces 10 and the balls 5 at an initial period (when setting the retaining piece 10 to the ball screw apparatus) inconveniently grow while using the ball screw apparatus. When the sum total in spaces of the retaining pieces 10 and the balls 5 increase and the spaces gather to one position, the retaining piece 10 falls as shown in FIG. 10 to probably cause bad working or earlier damage of the ball screw apparatus.
Japanese Patent Laid Open 21018/2001 discloses therefore a measure of interposing elastic materials between the adjacent balls together with the retaining pieces. Depending on this measure, if the ball is effected with high load on and off, it is possible to suppress abrasion and damage acting on the retaining pieces and the balls.
However, in the above measure, if the elastic material placed between the adjacent balls together with the retaining pieces is loaded, spring force of the elastic material acting on the loaded ball is powerless, and inconveniently a space adjusting capacity is not enough. Therefore, depending on space growth owing to abrasion and damage of the retaining piece and the ball, the retaining piece might fall, and there still remains a room for improvement.
In the above measure, an outer diameter of the retaining piece is set to be smaller than a diameter of the ball and this difference is determined to be larger than elastic deformation generated in the ball, but even the retaining pieces satisfying this condition might contact walls of the spiral ball rolling path or ball circulating path when passing them. Consequently, such inconvenience probably cause bad working or earlier damage of the ball screw apparatus, and there is still a room for determining sizes of the outer diameter of the retaining piece.